This invention relates to decoders for detecting and identifying coded tone signals appearing on a communication line, and deals more particularly with such a decoder comprised primarily of digital data processing components whereby the decoder may be implemented, if desired, by large scale integrated circuits or other solid state devices.
Tone code communication systems, as the term is used herein, are systems wherein a selected set of signals of distinctively different frequencies, and referred to as code tones, are used to communicate an associated set of informational bits. When a particular bit of information is to be communicated, the transmitting station inserts onto a communication line the code tone corresponding to such information bit. The receiving station must, therefore, necessarily include a decoder for detecting the presence of code tones on the communication line and for identifying each such detected tone. A presently common example of a tone coded communication system is a touch dial telephone communication system wherein coded tone pairs, generated by a touch dial pad, are used to communicate dialing formation.
In the past, tone decoders used in receiving stations have generally been based on a set of frequency responsive devices, one for each code tone and such as a set of narrow band pass filters, essentially analog in nature. These decoders, because of the number of frequency responsive devices required and because of the analog nature of such devices, have been relative expensive to produce and susceptible to malfunction due to departures in the characteristics of the components from desired characteristics because of initial manufacturing errors or change of such characteristics through age and use. Further, it has been impossible to implement such decoders by way of low cost and space saving integrated circuit components.
A general object of this invention is, therefore, to provide a tone decoder for use in a coded tone communication system for detecting and identifying coded tones and which decoder utilizes a minimum of analog or linear type circuit components and instead is comprised principally of components of the digital data processing type which may be made by integrated circuit or other solid state techniques.
A further object of this invention is to provide a tone decoder especially adapted for use in a touch dial telephone communication system or the like wherein communicated characters are transmitted by way of simultaneous tone pairs, the decoder utilizing principally digital data processing techniques and components.
Other objects and advantages of the invention will be apparent from the following description of a preferred embodiment of the same and from the drawings forming a part hereof.